Enhanced Oxidation Kinetics in Thermochemical Cycling of CeO2 through Templated Porosity

Two-step thermochemical cycling was achieved using CeO 2 with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO 2 . The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO 2 and nonordered macroporous (NOM) CeO 2 . Aggregated CeO 2 nanoparticles with feature sizes similar to those in 3DOM CeO 2 were prepared by fragmenting 3DOM CeO 2 into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO 2 were tested in thermochemical cycles using an infrared furnace. CeO 2 was reduced at ∼1200 °C, and the reduced CeO 2−δ materials were reoxidized under CO 2 at ∼850 °C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO 2 . Additionally, 3DOM CeO 2 provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling.